Restoring diversity after cattail expansion: disturbance, resilience, and seasonality in a tropical dry wetland.

As the human footprint expands, ecologists and resource managers are increasingly challenged to explain and manage abrupt ecosystem transformations (i.e., regime shifts). In this study, we investigated the role of a mechanical disturbance that has been used to restore and maintain local wetland diversity after a monotypic regime shift in northwestern Costa Rica [specifically, an abrupt landscape-scale cattail (Typha) expansion]. The study was conducted in Palo Verde Marsh (Palo Verde National Park; a RAMSAR Wetland of International Importance), a seasonally flooded freshwater wetland that has historically provided habitat for large populations of wading birds and waterfowl. A cattail (T. domingensis) expansion in the 1980s greatly altered the plant community and reduced avian habitat. Since then, Typha has been managed using a form of mechanical disturbance called fangueo (a Spanish word, pronounced "fahn-gay-yo" in English). We applied a Typha removal treatment at three levels (control, fangueo, and fangueo with fencing to exclude cattle grazing). Fangueo resulted in a large reduction in Typha dominance (i.e., decreased aboveground biomass, ramet density, and ramet height) and an increase in habitat heterogeneity. As in many ecosystems that have been defined by multiple and frequent disturbances, a large portion of the plant community regenerated after disturbance (via propagule banking) and fangueo resulted in a more diverse plant community that was strongly dictated by seasonal processes (i.e., distinct wet- and dry-season assemblages). Importantly, the mechanical disturbance had no apparent short-term impact on any of the soil properties we measured (including bulk density). Interestingly, low soil and foliar N:P values indicate that Palo Verde Marsh and other wetlands in the region may be nitrogen limited. Our results quantify how, in a cultural landscape where the historical disturbance regime has been altered and diversity has declined, a mechanical disturbance in combination with seasonal drought and flooding has been used to locally restrict a clonal monodominant plant expansion, create habitat heterogeneity, and maintain plant diversity.